Ardour does most of its internal signal routing via JACK: all track and bus inputs and outputs are JACK ports, as are sends and inserts, which means they can be tapped into by other JACK clients. Only the signal flow inside a track or bus (i.e. from processor to processor) is handled internally.
By default, Ardour will create the following connections for you:

• Track inputs are optionally auto-connected to hardware inputs, in round robin order, depending on the setting you chose in the Session > New Session dialog.
• Bus inputs are left disconnected.
• The number of track and bus outputs are equal to the number of inputs of the master bus.
• Track and bus outputs are always auto-connected to the master bus inputs.
• Master bus outputs are connected to hardware outputs.

This configuration is sufficient to do basic tracking and playback of many sessions without any adjustment by the user. If you are using Ardour and start to change these connections, be certain that you have a good reason to do so — it is generally not necessary and often leads to problems.

However, for many workflows during mixing, more complicated signal routing is required. Ardour offers many possibilties for connecting things in the way you may need or want them.